The invention relates to a programmable, non-volatile memory device (PROM), comprising a system of programmable non-volatile memory cells arranged in a matrix of rows and columns and provided with a first set of selection lines parallel to the columns and with a second set of selection lines parallel to the rows, a memory cell being associated with each point of intersection between the selection lines. The invention also relates to a method of manufacturing such a device.
Programmable semiconductor memories or PROMs are known in various shapes. One of the earliest PROM types made use of fuses, where programming of a selected cell implies that the connection between a word line and a bit lines is broken in that the fuse is melted. These memories may be readily manufactured by generally known i.c. techniques, but they have the disadvantage that information once written cannot be erased any more. This means that a new chip is to be used for writing new data. In addition, the cells themselves cannot be tested during production and it is necessary to provide extra test cells on the chip which cannot be utilized for the memory. Another type of programmable memories is known under names such as EEPROM, EPROM, Flash EPROM. Each memory cell here comprises an MOS transistor with floating gate. The information is written in the form of electric charge at the floating gate and thus determines the threshold voltage of the transistor. These memories are erasable in principle, which means that separate test cells are unnecessary. A disadvantage is that the memory cells are comparatively large, which renders it difficult to manufacture memories with a very large number of bits. A third type of programmable memories, also erasable, is based on a resistance change in materials upon the transition between the crystalline and the non-crystalline state. Such memories are known inter alia under the abbreviated designation MIM (Metal-Insulator-Metal). These memories require for each cell besides the MIM element also a selection element such as a transistor or a diode. In addition, a chalcogenide material which does not form part of standard silicon processes is often used for the switchable element.